Since ethylene/tetrafluoroethylene copolymers (ETFE) are excellent in heat resistance and weather resistance and lighter than glass, the use of an ETFE film on the surface of a solar battery, in replacement of glass, is now considered.
An ETFE film, however, problematically has insufficient transparency in comparison with glass. Moreover, if an ETFE film fails to be cooled in such a manner that the temperature distribution of the film becomes uniform upon production thereof by melt molding, the cooled film problematically has poor transparency and has loosening or wrinkles.
Patent Literature 1 discloses a method for producing a fluororesin film wherein, upon production of a flat film by contacting a molten fluororesin extruded from a T-die with a cooling roll and then rolling the cooled and solidified resin up, the cooling roll is set to have a surface temperature of 80° C. to 140° C. and hot air at 50° C. to 160° C. is blown to the film on the cooling roll, for the purpose of producing a fluororesin film excellent in flatness and optical properties such as transparency and gloss.
Patent Literature 2 discloses, as a tetrafluoroethylene-ethylene copolymer having a high light transmittance and low haze, a tetrafluoroethylene-ethylene copolymer wherein the copolymer having a ratio of (polymerization unit derived from tetrafluoroethylene)/(polymerization unit derived from ethylene) of 30/70 to 70/30 (molar ratio) further contains 1 to 10 (mol %) of a polymerization unit derived from vinyl ether represented by CF2═CF—O—R (1) (in the formula, R represents C3-C12 alkyl group optionally containing 1 to 3 ether oxygen atoms), is crystalline, and has a volume flow rate of 1 to 1000 mm3/sec.
Patent Literature 3 discloses a biaxially stretched film such as an ethylene-tetrafluoroethylene-based copolymer film having significantly improved light transmittance.